1. Field of the Invention
The present invention relates to a metallic cord for reinforcing a tire and a pneumatic tire employing the metallic cord for a belt layer and/or a carcass.
2. Description of the Prior Art
Important subjects for a pneumatic radial tire, particularly a radial tire for a passenger car are reduction of rolling resistance, improvement of durability, reduction of the manufacturing cost, and improvement in comfortableness of the vehicle.
The characteristics of a belt layer, particularly those of a cord embedded in the belt layer and the rigidity of the belt layer exert remarkable influence on improvement of the aforementioned performance.
For example, Japanese Utility Model Laying-Open No. 63-19404 (1988) proposes a technique of reducing the rolling resistance of a tire by employing not a cord but a steel wire material for a belt layer. In such a tire, however, the steel wire material is disadvantageously broken when a vehicle provided with such a tire repeats steep turns.
The rigidity of the belt layer dominates the deformation of the belt layer when a vehicle provided with the tire is in motion, and influences the basic performance and durability of the tire. In particular, the rigidity of the belt layer influences the rolling resistance, high-speed driving performance and cornering performance as well as belt end separation resulting from cracking between plies at an end portion of the belt layer caused by a load applied to the tire when the car is in motion. Therefore, prescribed rigidity of the belt layer must be ensured. The belt end separation is caused by growth of cracking of rubber resulting from stress concentrating on the end portion of the belt layer due to repetitive deformation of the belt layer when the car provided with the tire is in motion, particularly deformation of the belt layer upon cornering. Therefore, resistance against such belt end separation is important for attaining durability of the tire.
On the other hand, a pneumatic radial tire for a truck or a bus used under sever conditions due to a high load and high-speed driving must maintain at high levels the strength of not only a cord for a belt layer but also a carcass ply cord. In general, the carcass of such a radial tire for a truck or a bus must be improved in transverse rigidity in order to maintain steering stability when subjected to repetitive deformation following high-speed driving under a high load. Therefore, the carcass cord is formed by a steel cord, in order to satisfy requirement for strength and durability for sufficiently withstanding such distortion/deformation.
In general, a single-layer stranded structure of a multilayer stranded structure of 7×4, 3+7 or 3+8+13 is employed for the carcass cord of this type of tire. When such a cord densely charged with steel wires is embedded in carcass rubber, however, the rubber insufficiently penetrates into the steel cord. Therefore, when a side wall portion is damaged by sharp stones and rocks during driving, for example, it follows that water penetrates the damaged portion, which in turn serves as a starting point for diffusing moisture into the space defined by the steel cord, causing the creation and spreading of rust. Consequently, the adhesion between the steel cord and the rubber deteriorates and the steel cord becomes broken which reduces the durability of the tire.
Further, the aforementioned steel cord, having a complicated structure, requires complicated manufacturing steps, which results in a high manufacturing cost for the metallic cord.
In order to solve these problems, there is proposed the so-called loose cord construction formed by stranding steel wires while defining a clearance therebetween to produce steel cords containing a clearance between the wires for improving the penetrability of rubber into the cord. However, such a steel cord requires a forming step as well as a stranding step for the steel wires resulting in a high manufacturing cost for the steel cord. Also, the steel wires come apart from each other in the step of molding the tire, damaging the uniformity and durability of the product tire.
In order to solve this problem, each of Japanese Patent Laying-Open Nos. 10-292275 (1998) and 10-292276 (1998), for example, employs an unstranded metallic cord. However, cord wire materials forming the unstranded metallic cord, not shaped as a cord disadvantageously separate from each other in the step of molding the tire, resulting in various problems.
Japanese Patent Laying-Open No. 11-27446 (1999) discloses an example of binding unstranded cords with a wrapping wire. However, the wrapping wire bundling and shaping a plurality of steel wires locally rubs the steel wires to reduce durability. Prevention of such rubbing is limited since shaping performance is reduced when the cords are loosely bundled.